The present invention relates to a printed circuit board made of laminate for electric circuits, comprising at least one carrier layer and at least one surface layer made of copper intended, after the removal of selected portions, for example by etching, to constitute the conductors on the board. The invention also relates to a method for producing printed circuit board laminate of the abovementioned type intended for electric circuits.
Circuit boards with conductors consisting of a copper layer etched into a pattern located on one or both sides of the board or even inside the board are commonly used within the field of electronics. These boards often have a base layer consisting of glass fibre mixed with epoxy. The glass content in these mixtures can vary, but common values are 40-60%. One problem, especially with an advanced degree of miniaturization, is thermal expansion which can result in breakage and contact difficulties. Another problem is surface roughness. If the quantity of glass is increased, the thermal expansion coefficient decreases, but a disadvantage of this is that the surface of the laminate becomes coarser. A secondary consequence of the surface becoming coarser, that is to say being imparted an inferior surface finish, is that, when copper plating is applied, it is not possible to have too thin a copper layer because the conductor pattern is jeopardized by the coarse surface. On the other hand, with a thicker copper layer, it must be borne in mind that etching has to continue until those portions which are to be removed are completely etched through and removed, which means that the pattern cannot be made too fine-meshed because the etching becomes more uneven at the edges as the etching time increases and the risk of defects then increases. There are circuit boards which have a low thermal expansion coefficient but these are in most cases based on cores of metal or ceramics and are relatively expensive.
By means of the present invention, a circuit board has been produced, in which the material makes it possible for the linear expansion due to temperature increases to be reduced considerably, but in which it is still possible to have relatively thin copper layers and thus a fine-meshed pattern with conductor widths as small as 50 xcexcm. The board is still inexpensive in comparison with, for example, the variants with the metal cores. To this end, the invention is characterized in that the carrier layer has, at least in certain portions, been allowed to have a surface roughness of up to essentially the same size as the thickness of the copper layer, and in that, at least in the abovementioned portions, a surface-levelling plane layer is arranged between the carrier layer and the copper layer. The plane layer suitably consists of epoxy and has a thickness which is essentially the same size as the surface roughness of the carrier layer, that is to say is essentially the same thickness as the surface roughness of the carrier layer. In this connection, a carrier layer which contains glass fibre and epoxy can advantageously be used, and good linear expansion properties are achieved if the glass fibre content exceeds 70%.
The epoxy layer levels the surface of the coarse carrier layer and increases the adhesion between the copper and the glass fibre, so that a thinner copper layer can be used on the surface thus smoothed, which in turn results in shorter etching time and less unevenness at the edges of the etching, with the possibility of narrow conductors.
The new method is characterized in that a layer of liquid epoxy is applied to that surface or those surfaces of the carrier layer which has or have a surface roughness of the same order of size as the thickness of the copper layer envisaged and which in the end is or are to carry a copper layer, in that the epoxy layers are dried and hardened, and in that copper layers are plated on top of the epoxy layers.
In the case of those laminates which are to have through-plated holes, it is an advantage if these holes are drilled after application of epoxy but before the copper-plating because it is then possible to obtain a homogeneous copper thickness.